Method of polishing end face of multi-fiber optical connector

ABSTRACT

To provide a method of polishing an end face of a multi-fiber optical connector capable of eliminating recesses produced in core parts of multi-mode optical fibers. 
     A polishing base is prepared, comprising a soft material having a Shore hardness of less than 30 and a film having a thickness of less than 75 μm, without including any polishing material, on said soft material, and further comprising a polishing material on said film. Next, a multi-fiber optical connector having a plurality of multi-mode optical fibers is arranged so that the tip of the multi-mode optical fibers come into contact with the upper surface of the film with a predetermined pressure. Next, the tip of the multi-mode optical fibers are polished by moving at least one of the polishing base and the multi-fiber optical connector while maintaining the contact between the tip of the multi-mode optical fibers and the upper surface of the film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-060908, filed in Japan on Mar. 9, 2007, and the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of polishing an end face of a multi-fiber optical connector for polishing an end face of a plurality of optical fibers fixed on a multi-fiber optical connector by adhesion, in particular, a multi-mode fiber (MM fiber).

2. Description of the Related Art

In general, in a multi-fiber optical connector, a plurality of or many optical fibers are aligned inside a ferrule and fixed thereon by adhesion. The tip part of the optical fiber projects by a predetermined length outwardly from the end face for connection of the ferrule. The identical multi-fiber optical connectors are arranged so that the end faces for connection of the ferrule face each other and then the tip parts of the optical fibers are optically connected. Due to this, high speed transmission of a large amount of data can be realized.

Such a multi-fiber optical connector is manufactured, for example, as follows. First, a plurality of optical fibers are aligned inside a ferule made of resin and fixed by adhesion. Next, the end face for connection of the ferrule is polished into a flat surface (surface polishing step). Subsequently, the end face for connection of the ferrule is buffed using a fine-grain abrasive. Due to this, the tip part of the optical fiber projects by a predetermined length outwardly from the end face for connection of the ferrule (projection step).

Conventional methods of polishing an end face used for manufacturing a multi-fiber optical connector have been disclosed in, for example, Japanese Patent Application Laid-open Nos. H10-48467, H8-126951, and 2003-334749.

However, in the above manufacturing of the multi-fiber optical connector, in the projection step after the surface polishing step, the projection length of the optical fiber by which it projects outwardly from the end face for connection of the ferrule varies from fiber to fiber. In particular, among the plurality of optical fibers aligned on the end face for connection of the ferrule, the optical fibers located on both sides project less than other optical fibers located at portions other than both sides.

The reason for that is as follows. Each optical fiber located at a portion other than both sides is between adjacent optical fibers. Because of this, it is unlikely that the optical fiber is polished so excessively that the projection length of the optical fiber becomes shorter than the projection length of the optical fibers on both sides. In contrast to this, one of the optical fibers located on both sides has only one adjoining optical fiber on one of the sides. On the other opposite side, there is no optical fiber but only a resin material constituting the ferrule. Because of this, it is likely that the optical fibers located on both sides are polished excessively compared to the optical fiber located on a portion other than both sides and as a result, the length of the optical fibers located on both sides is shortened.

If such multi-fiber optical connectors are connected optically, it is unavoidable that the contact force between the optical fibers located on both sides is weakened.

In the case of a normal quartz-base optical fiber, the hardness of the core part is less than that of the clad part. Because of this, there is a tendency that a recess is likely to be produced in the tip of the core part by the buffing in the projection step in the manufacture of a multi-fiber optical connector.

When, however, the optical fiber is a single-mode fiber (SM fiber), it is possible to substantially ignore a recess produced in the tip of the core part. The single-mode fiber is constituted by a clad part having a diameter of, for example, about 125 μm and a core part located substantially in the center of the clad part and having a diameter of, for example, about 8 μm. As described above, the diameter of the core part of the single-mode fiber is very small compared to the diameter of the clad part. Because of this, in the case of a single-mode fiber, a recess produced in the core part by buffing in the projection process is very small and the depth thereof from the tip is very small.

Because of this, in the case of a single-mode fiber, there arises no problem of connection loss in the optical connection between the above-mentioned multi-fiber optical connectors. That the contact force between optical fibers (SM fibers) is weak does not cause a gap to be produced between the tips of the optical fibers (SM fibers). Because of this, it is unlikely that the connection loss between optical fibers (SM fibers), in particular, the return loss, is affected substantially by the recess in the core part.

In contrast to this, the multi-mode fiber (MM fiber) is constituted by a clad part having a diameter of, for example, about 125 μm and a core part located substantially in the center of the clad part and having a diameter of, for example, about 50 μm or 62.5 μm. As described above, the diameter of the core part of the multi-mode fiber is by far larger than that of the single-mode fiber. Because of this, in the case of the multi-mode fiber, the recess produced in the core part by buffing in the projection process is by far larger than that in the case of a single-mode fiber. That is, in the tip of the core part of a multi-mode fiber, a large and deep recess is produced.

Because of this, in the case of a multi-mode fiber, there arises a problem of connection loss in the optical connection between the multi-fiber optical connectors described above. That the contact force between optical fibers (MM fibers) is weak causes a gap to be produced between tips of optical fibers (MM fibers). Because of this, the connection loss between optical fibers (MM fibers), in particular, the return loss, increases.

SUMMARY OF THE INVENTION

In order to solve the above problem, an object of the present invention is to provide a method of polishing an end face of a multi-fiber optical connector capable of eliminating recesses produced in core parts of multi-mode optical fibers.

To achieve the above object, one aspect of the present invention provides a method of polishing an end face of a multi-fiber optical connector, the method comprising:

a first step of preparing a polishing base, comprising a soft material having a Shore hardness of less than 30 and a film having a thickness of less than 75 μm, without including any polishing material, on said soft material, and further comprising a polishing material on said film;

a second step of arranging a multi-fiber optical connector having a plurality of multi-mode optical fibers so that the tip ends of the multi-mode optical fibers come into contact with an upper surface of the film with a predetermined pressure; and

a third step of polishing the tip ends of the multi-mode optical fibers by moving at least one of the polishing base and the multi-fiber optical connector while maintaining the contact between the tip ends of the multi-mode optical fibers and the upper surface of the film.

Preferably, the soft material is composed of a spongy porous substance.

Preferably, at least the upper surface of the film is made coarse by a surface processing.

Preferably, the polishing material is applied to the coarsened upper surface of the film.

Preferably, the soft material is a sponge pad.

Preferably, the soft material has a thickness of approximately 5 mm.

Preferably, the film is a polyethylene terephthalate film (PET film).

Preferably, the PET film has a thickness of approximately 25 μm.

Preferably, the polishing material has an average grain diameter of 0.5 μm or less.

Another aspect of the present invention provides a method of polishing an end face of a multi-fiber optical connector, the method including:

a first step of mounting a sponge pad having a Shore hardness of less than 30 on a polishing base of a polishing machine;

a second step of mounting a polyethylene terephthalate film (PET film) having a thickness of less than 75 μm without any polishing material on the sponge pad;

a third step of supplying an polishing material on the upper surface of the PET film;

a fourth step of arranging a multi-fiber optical connector having a plurality of multi-mode optical fibers so that the tip ends of the multi-mode optical fibers come into contact with an upper surface of the film with a predetermined pressure; and

a fifth step of polishing the tip ends of the multi-mode optical fibers by moving at least one of the polishing base and the multi-fiber optical connector while maintaining the contact between the tip ends of the multi-mode optical fibers and the upper surface of the PET film.

Preferably, the sponge pad has a thickness of approximately 5 mm.

Preferably, the PET film has a thickness of approximately 25 μm.

Preferably, the polishing material has an average grain diameter of approximately 0.5 μm or less.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

These and other objects and the configuration of this invention will become clearer from the following description of the preferred embodiments, read in connection with the accompanying drawings in which:

FIG. 1 is a schematic perspective view showing an embodiment of a multi-fiber optical connector used in a method of polishing an end face according to the present invention;

FIG. 2A is a schematic enlarged end face view of a multi-mode optical fiber used in a multi-fiber optical connector according to the present invention;

FIG. 2B is a schematic enlarged sectional view of a multi-mode optical fiber used in a multi-fiber optical connector according to the present invention;

FIG. 3A is a schematic explanatory diagram of relevant parts showing a state when an adhesive removal step is started, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 3B is a schematic explanatory diagram of relevant parts showing a state after the adhesive removal step is completed, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 4A is a schematic explanatory diagram of relevant parts showing a state when a surface polishing step is started, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 4B is a schematic explanatory diagram of relevant parts showing a state when the surface polishing step is completed, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 5A is a schematic explanatory diagram of relevant parts showing a state when a projection step is started, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 5B is a schematic explanatory diagram of relevant parts showing a state when the projection step is completed, the shape and dimensions of each part being shown exaggerated for easier understanding;

FIG. 6 is a schematic enlarged sectional view of a multi-mode optical fiber after the projection step;

FIG. 7 is a schematic explanatory diagram of relevant parts showing a recess elimination step in a core part, the shape and dimensions of each part being shown exaggerated for easier understanding; and

FIG. 8 is a schematic enlarged sectional view of a multi-mode optical fiber after the recess elimination step in the core part.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic perspective view showing an embodiment of a multi-fiber optical connector used in a method of polishing an end face according to the present invention, and FIGS. 2A and 2B are a schematic enlarged end face view and an enlarged sectional view of a multi-mode optical fiber used in a multi-fiber optical connector according to the present invention, respectively.

As shown in FIG. 1, a multi-fiber optical connector 10 is a connector of MT (Mechanically Transferable) type, including a rectangular parallelepiped ferrule 20. A plurality of or many optical fibers 30 are aligned in the ferrule 20 and fixed by adhesion. A tip part 35 of the optical fiber projects by a predetermined length outwardly from an end face 25 for connection of the ferrule 20.

The ferrule 20 includes optical fiber insertion holes 21 in which each of the optical fibers 30 is arranged. The tip of the optical fiber insertion holes 21 open in the end face 25 for connection of the ferrule 20.

In one of the side faces of the ferrule 20, in the upper side face in FIG. 1, an adhesive injection hole 22 is formed. The adhesive injection hole 22 is communicated with the optical fiber insertion holes 21. The optical fibers 30 are arranged in the optical fiber insertion holes 21 and an adhesive 40 is injected through the adhesive injection hole 22. Due to this, the optical fibers 30 are fixed on the ferrule 20 by adhesion.

The multi-fiber optical connector 10 is optically connected with the identical multi-fiber optical connector, both facing each other. Because of this, guide pin insertion holes 23 are formed in the end face 25 for connection of the ferrule 20. When the multi-fiber optical connectors 10 are optically connected, guide pins (not shown) for positioning are inserted into the guide pin insertion holes 23.

It is possible to compose such a ferrule 20 using, for example, an epoxy base resin material containing glass.

As shown in FIGS. 2A and 2B, the optical fiber 30 is constituted by a clad part 31 and a core part 32 located substantially in the center of the clad part 31. A diameter D1 of the clad part 31 is, for example, about 125 μm and a diameter D2 of the core part 32 is, for example, about 50 μm or 62.5 μm. The optical fiber 30 is a so-called multi-mode optical fiber.

A method of polishing an end face of the multi-fiber optical connector 10 as described above is explained next.

A plurality of or many multi-mode optical fibers 30 are arranged in the optical fiber insertion holes 21 of the ferrule 20. Next, the adhesive 40 is injected through the adhesion injection hole 22. Due to this, the optical fibers 30 are fixed on the ferrule 20 by adhesion. That is, the multi-fiber optical connector 10 is constructed.

At this time, the adhesive 40 injected through the adhesive injection hole 22 passes through the optical fiber insertion holes 21 and overflows from the tip openings. The adhesive 40 having overflowed covers the tip part 35 of the optical fibers that project outwardly from the end face 25 for connection of the ferrule 20.

First of all, a step of removing adhesive of the multi-fiber optical connector 10 is carried out. As shown in FIGS. 3A and 3B, the adhesive 40 that covers the tip parts 35 of the optical fibers projecting outwardly from the end face 25 for connection of the ferrule 20 is removed. FIGS. 3A and 3B schematically show the relevant parts of the multi-fiber optical connector 10. Because of this, the shape and dimensions of each part are shown exaggerated for easier understanding.

As shown in FIG. 3A, a rubber pad 51 is mounted on a polishing base (basement) 50 of a polishing machine. On the rubber pad 51, a polishing sheet 52 is mounted. The multi-fiber optical connector 10 is arranged so that the tip parts 35 of the optical fibers come into contact or the adhesive 40 that covers the tip parts 35 comes into contact with the upper surface of the polishing sheet 52 with a predetermined pressure. In a state where the contact is maintained, the polishing base 50 moves so that the center axis line of the polishing base 50 describes a circular locus while keeping a rotation-preventing attitude. Alternatively, the multi-fiber optical connector 10 moves so that the center axis line of the multi-fiber optical connector 10 describes a circular locus while keeping a rotation-preventing attitude. By the movement of the polishing base 50 or the multi-fiber optical connector 10, the adhesive 40 that covers the tip parts 35 of the optical fibers are polished. Due to this, the adhesive 40 is removed.

At this time, the tip parts 35 of the optical fibers are pressed against the polishing sheet 52 by a comparatively strong force. This force can be weakened by the deformation of the rubber pad 51. Accordingly, excessive load is not put on the optical fibers 30. Therefore, it is possible to prevent cracks in the multi-mode optical fibers 30 and to remove the adhesive 40 properly.

FIG. 3B shows a state where the adhesive 40 has been removed.

Next, a surface polishing step of the multi-fiber optical connector 10 is carried out. As shown in FIGS. 4A and 4B, the end face 25 for connection of the ferrule 20 is polished into a flat surface. FIGS. 4A and 4B schematically show the relevant parts of the multi-fiber optical connector 10. Because of this, the shape and dimensions of each part are shown exaggerated for easier understanding.

As shown in FIG. 4A, the polishing sheet 52 is mounted on the polishing base (basement) 50 of the polishing machine. The multi-fiber optical connector 10 is arranged so that the tip parts 35 of the optical fibers come into contact with the upper surface of the polishing sheet 52 with a predetermined pressure. In a state where the contact is maintained, the polishing base 50 moves so that the center axis line of the polishing base 50 describes a circular locus while keeping a rotation-preventing attitude. Alternatively, the multi-fiber optical connector 10 moves so that the center axis line of the multi-fiber optical connector 10 describes a circular locus while keeping a rotation-preventing attitude. By the movement of the polishing base 50 or the multi-fiber optical connector 10, the tip parts 35 of the multi-mode optical fibers 30 that project from the end face 25 for connection of the ferrule 20 are polished. Accordingly, the tip parts 35 of the multi-mode optical fibers 30 that project from the end face 25 for connection of the ferrule 20 are completely removed and the end face 25 for connection of the ferrule 20 becomes almost flat.

The state when the surface polishing step is completed is shown in FIG. 4B.

Next, the projection step of the multi-fiber optical connector 10 is carried out. As shown in FIGS. 5A and 5B, the tip parts 35 of the multi-mode optical fibers 30 project by a predetermined length from the end face for connection of the ferrule 20. FIGS. 5A and 5B schematically show the relevant parts of the multi-fiber optical connector 10. Because of this, the shape and dimensions of each part are shown exaggerated for easier understanding.

As shown in FIG. 5A, a polishing sheet 53 for buffing using a fine-grain abrasive is mounted on the polishing base (basement) 50 of the polishing machine. The multi-fiber optical connector 10 is arranged so that the end face 25 for connection of the ferrule 20 comes into contact with the upper surface of the polishing sheet 53 with a predetermined pressure. In a state where the contact is maintained, the polishing base 50 moves so that the center axis line of the polishing base 50 describes a circular locus while keeping a rotation-preventing attitude. Alternatively, the multi-fiber optical connector 10 moves so that the center axis line of the multi-fiber optical connector 10 describes a circular locus while keeping a rotation-preventing attitude. By the movement of the polishing base 50 or the multi-fiber optical connector 10, the end face 25 for connection of the ferrule 20 is polished. Due to this, part of the ferrule is removed.

As a result, the tip parts 35 of the multi-mode optical fibers 30 project by a predetermined length from the end face 25 for connection of the ferrule 20.

The state when the projection step is completed is shown in FIG. 5B.

FIG. 6 is an enlarged sectional view of the tip part 35 of one multi-mode optical fiber 30 after the multi-fiber optical connector 10 shown in FIG. 5B is reversed upside down.

In the case of the multi-mode optical fiber 30, the diameter of the core part 32 is by far larger than that of the SM fiber. The hardness of the core part 32 is less than that of the clad part 31. Because of this, by buffing in the projection process, a large and deep recess is produced in the tip of the core part 32 as shown in FIG. 6.

Such a large and deep recess 33 brings about a problem when the identical multi-fiber optical connectors 10 are connected optically. That the contact force between the multi-mode optical fibers 30 is weak substantially causes a gap to be produced between the tips of the multi-mode optical fibers 30. Such a gap considerably increases connection loss, in particular, return loss, in the optical connection between the multi-fiber optical connectors 10.

In order to solve this problem, the recess 33 produced in the core part 32 of the multi-mode optical fiber 30 is eliminated as shown in FIG. 7. FIG. 7 schematically shows relevant parts of the multi-fiber optical connector 10. Because of this, the shape and dimensions of each part are shown exaggerated for easier understanding.

As shown in FIG. 7, a soft material 55 having a Shore hardness of less than 30 is mounted on the polishing base (basement) 50 of the polishing machine. On the soft material 55, a film 56 having a thickness of less than 75 μm without a polishing material is mounted. A polishing material (abrasive grain) is supplied on the upper surface of the film 56. The multi-fiber optical connector 10 is arranged so that the tip parts 35 of the optical fibers come into contact with the upper surface of the film 56 with a predetermined pressure. In a state where the contact is maintained, the polishing base 50 moves so that the center axis line of the polishing base 50 describes a circular locus while keeping a rotation-preventing attitude. Alternatively, the multi-fiber optical connector 10 moves so that the center axis line of the multi-fiber optical connector 10 describes a circular locus while keeping a rotation-preventing attitude. By the movement of the polishing base 50 or the multi-fiber optical connector 10, the tip ends of the multi-mode optical fibers 30 are polished. Due to this, the recesses 33 in the tip of the core parts 32 of the optical fibers (MM fibers) 30 are removed.

The soft material 55 is composed of, for example, a spongy porous substance. The soft material (soft pad) 55 has a Shore hardness of less than 30. Actually, it is preferable that the soft material (soft pad) 55 be very soft as a sponge. In addition, it is preferable that the soft material (sponge pad) 55 has a thickness of, for example, about several millimeters.

The film 56 is composed of, for example, a polyethylene terephthalate film (PET film) at least one side of which has been primary-coated. The primary coating here refers to making coarse by surface processing. The thickness of the film (PET film) 56 is less than 75 μm. Actually, it is preferable that the thickness of the film (PET film) 56 be about 25 μm. It is also preferable that the film (PET film) 56 be placed with the primary-coated surface (coarsened surface) face-up. The polishing material (polishing grain) 60 is supplied to the surface (coarsened surface).

It is preferable to use a fine-grain abrasive (abrasive grain) as the polishing material (polishing grain) 60. Here, a fine-grain abrasive refers to a polishing material (polishing grain) having an average grain diameter of 0.5 μm or less. Preferably, the polishing material (polishing grain) 60 is applied to the primary-coated surface (coarsened surface) of the film 56.

By the polishing shown in FIG. 7, the recesses 33 in the tip of the core parts 32 of the multi-mode optical fibers 30 are eliminated.

FIG. 8 is an enlarged sectional view of the tip part 35 of the multi-mode optical fiber 30 for which the polishing shown in FIG. 7 has been completed. It can be seen that not only the recess 33 in the core part 32 is eliminated from the tip of the multi-mode optical fiber 30 but also the tip is formed into a convex spherical surface with the core part 32 as its vertex. The polishing shown in FIG. 7 polishes the tip of the multi-mode optical fibers into a convex spherical surfaces with the core parts 32 as its vertex.

Example 1

On the polishing base 50 of the polishing machine, the sponge (sponge pad) 55 having a Shore hardness of less than 30 and a thickness of 5 mm was mounted. On the sponge pad 55, the polyethylene terephthalate film (PET film) 56 having a thickness of 25 μm and one side of which had been primary-coated (coarsened by surface processing) was mounted. The PET film 56 was placed with the primary-coated surface (coarsened surface) face-up. The fine-grain abrasive (abrasive grain) 60 was applied to the upper surface (primary-coated surface) of the PET film 56.

The multi-fiber optical connector 10 was arranged so that the tip parts 35 of the multi-mode optical fibers 30 came into contact with the upper surface of the PET film 56 with a predetermined pressure.

In a state where each of the contact between the tip of the multi-mode optical fibers 30 and the upper surface of the PET film 56 was maintained, the polishing base 50 moved so that the center axis line of the polishing base 50 described a circular locus while keeping a rotation-preventing attitude.

As a result, in the multi-mode optical fibers 30, the tip of the clad parts 31 that projected more than the tip of the core parts 32 were polished first. As a result, the recesses 33 in the core parts 32 were eliminated.

Alternatively, instead of the polishing base 50 left at rest, the multi-fiber optical connector 10 moved so that the center axis line of the multi-fiber optical connector 10 described a circular locus while keeping a rotation-preventing attitude.

In this case also, in the multi-mode optical fibers 30, the tip of the clad parts 31 that projected more than the tip of the core parts 32 were polished first. As a result, the recesses 33 in the core parts 32 were eliminated.

When either of the polishing base 50 or the multi-fiber optical connector 10 moved, a convex spherical surfaces with the core parts 32 as its vertex were formed at the tip of the multi-mode optical fibers 30 of the obtained multi-fiber optical connector 10.

When the multi-fiber optical connectors 10 were connected optically, both connection loss and return loss fell in a predetermined permitted range. In particular, the return loss was equal to or more than 40 dB and an excellent result was obtained.

Comparative Example 1

On the polishing base 50 of the polishing machine, a rubber pad having a Shore hardness of 30 and a thickness of 5 mm was mounted. On the rubber pad, the polyethylene terephthalate film (PET film) 56 having a thickness of 25 μm and one side of which had been primary-coated (coarsened by surface processing) was mounted. The PET film 56 was placed with the primary-coated surface (coarsened surface) face-up. The fine-grain abrasive (abrasive grain) 60 was applied to the upper surface (primary-coated surface) of the PET film 56.

The multi-fiber optical connector 10 was arranged so that the tip parts 35 of the multi-mode optical fibers 30 came into contact with the upper surface of the PET film 56 with a predetermined pressure.

In a state where each of the contact between the tip of the multi-mode optical fibers 30 and the upper surface of the PET film 56 was maintained, the polishing base 50 moved so that the center axis line of the polishing base 50 described a circular locus while keeping a rotation-preventing attitude.

However, in the multi-mode optical fibers 30, the recesses 33 produced in the tip of the core parts 32 in the projection step were not eliminated completely but a portion of them was left.

Alternatively, instead of the polishing base 50 left at rest, the multi-fiber optical connector 10 moved so that the center axis line of the multi-fiber optical connector 10 described a circular locus while keeping a rotation-preventing attitude.

In this case also, in the multi-mode optical fibers 30, the recesses 33 in the tip of the core parts 32 were not eliminated completely but a portion of them was left.

When either of the polishing base 50 or the multi-fiber optical connector 10 moved, part of the recess 33 in the core part 32 was left in the tip of the multi-mode optical fiber 30 of the obtained multi-fiber optical connector 10.

When the multi-fiber optical connectors 10 were connected optically, both connection loss and return loss varied considerably. In particular, the return loss varied in the range of 15 to 35 dB and some deviated from a predetermined permitted range.

Example 2

On the polishing base 50 of the polishing machine, the sponge (sponge pad) 55 having a Shore hardness of less than 30 and a thickness of 5 mm was mounted. On the sponge pad 55, the polyethylene terephthalate film (PET film) 56 having a thickness of 75 μm and one side of which had been primary-coated (coarsened by surface processing) was mounted. The PET film 56 was placed with the primary-coated surface (coarsened surface) face-up. The fine-grain abrasive (abrasive grain) 60 was applied to the upper surface (primary-coated surface) of the PET film 56.

The multi-fiber optical connector 10 was arranged so that the tip parts 35 of the multi-mode optical fibers 30 came into contact with the upper surface of the PET film 56 with a predetermined pressure.

In a state where each of the contact between the tip of the multi-mode optical fibers 30 and the upper surface of the PET film 56 was maintained, the polishing base 50 moved so that the center axis line of the polishing base 50 described a circular locus while keeping a rotation-preventing attitude.

As a result, in the multi-mode optical fibers 30, the tip of the clad parts 31 that projected more than the tip of the core parts 32 were polished first. As a result, the recesses 33 in the core parts 32 were eliminated.

Alternatively, instead of the polishing base 50 left at rest, the multi-fiber optical connector 10 moved so that the center axis line of the multi-fiber optical connector 10 described a circular locus while keeping a rotation-preventing attitude.

In this case also, in the multi-mode optical fibers 30, the tip of the clad parts 31 that projected more than the tip of the core parts 32 were polished first. As a result, the recesses 33 in the core parts 32 were eliminated.

When either of the polishing base 50 or the multi-fiber optical connector 10 moved, a convex spherical surfaces with the core parts 32 as its vertex were formed at the tip of the multi-mode optical fibers 30 of the obtained multi-fiber optical connector 10.

When the multi-fiber optical connectors 10 were connected optically, both connection loss and return loss fell in a predetermined permitted range. In particular, the return loss was equal to or more than 35 dB and an excellent result was obtained.

Comparative Example 2

On the polishing base 50 of the polishing machine, the sponge (sponge pad) 55 having a Shore hardness of less than 30 and a thickness of 5 mm was mounted. On the sponge pad 55, the polyethylene terephthalate film (PET film) 56 having a thickness of 75 μm and containing a polishing material (abrasive grain) was mounted.

The multi-fiber optical connector 10 was arranged so that the tip parts 35 of the multi-mode optical fibers 30 came into contact with the upper surface of the PET film 56 with a predetermined pressure.

In a state where each of the contact between the tip of the multi-mode optical fibers 30 and the upper surface of the PET film 56 was maintained, the polishing base 50 moved so that the center axis line of the polishing base 50 described a circular locus while keeping a rotation-preventing attitude.

However, in the multi-mode optical fibers 30, the recesses 33 produced in the tip of the core parts 32 in the projection step were not eliminated completely but a portion of them was left.

Alternatively, instead of the polishing base 50 left at rest, the multi-fiber optical connector 10 moved so that the center axis line of the multi-fiber optical connector 10 described a circular locus while keeping a rotation-preventing attitude.

In this case also, in the multi-mode optical fibers 30, the recesses 33 in the tip of the core parts 32 were not eliminated completely but a portion of them was left.

When either of the polishing base 50 or the multi-fiber optical connector 10 moved, part of the recess 33 in the core part 32 was left in the tip of the multi-mode optical fiber 30 of the obtained multi-fiber optical connector 10.

When the multi-fiber optical connectors 10 were connected optically, both connection loss and return loss varied considerably. In particular, the return loss varied in the range of 15 to 35 dB and some deviated from a predetermined permitted range.

Comparative Example 3

On the polishing base 50 of the polishing machine, a rubber pad having a Shore hardness of 80 and a thickness of 5 mm was mounted. On the rubber pad, the polyethylene terephthalate film (PET film) 56 having a thickness of 75 μm and containing a polishing material (abrasive grain) was mounted.

The multi-fiber optical connector 10 was arranged so that the tip parts 35 of the multi-mode optical fibers 30 came into contact with the upper surface of the PET film 56 with a predetermined pressure.

In a state where each of the contact between the tip of the multi-mode optical fibers 30 and the upper surface of the PET film 56 was maintained, the polishing base 50 moved so that the center axis line of the polishing base 50 described a circular locus while keeping a rotation-preventing attitude.

However, in the multi-mode optical fibers 30, the recesses 33 produced in the tip of the core parts 32 in the projection step were hardly eliminated and remained substantially the same as that before polishing.

Alternatively, instead of the polishing base 50 left at rest, the multi-fiber optical connector 10 moved so that the center axis line of the multi-fiber optical connector 10 described a circular locus while keeping a rotation-preventing attitude.

In this case also, in the multi-mode optical fibers 30, the recesses 33 in the tip of the core parts 32 was hardly eliminated and remained substantially the same as that before polishing.

When either of the polishing base 50 and the multi-fiber optical connector 10 moved, the recesses 33 in the core parts 32 remained substantially the same as that before polishing in the tip of the multi-mode optical fibers 30 of the obtained multi-fiber optical connector 10.

When the multi-fiber optical connectors 10 were connected optically, both connection loss and return loss deviated considerably from a predetermined permitted range. In particular, the return loss was 15 dB or less, considerably lower than a predetermined permitted range.

According to the present invention, it is possible to effectively eliminate recesses produced in the tip of the core parts 32 of the multi-mode optical fibers 30 in the projection step following the surface polishing step of the multi-fiber optical connector 10.

In addition to the above, a convex spherical surfaces with the core parts 32 as its vertex are formed at the tip of the multi-mode optical fibers 30 of the multi-fiber optical connector 10. 

1. A method of polishing an end face of a multi-fiber optical connector, the method comprising: a first step of preparing a polishing base, comprising a soft material having a Shore hardness of less than 30 and a film having a thickness of less than 75 μm, without including any polishing material, on said soft material, and further comprising a polishing material on said film; a second step of arranging a multi-fiber optical connector having a plurality of multi-mode optical fibers so that the tip ends of the multi-mode optical fibers come into contact with an upper surface of the film with a predetermined pressure; and a third step of polishing the tip ends of the multi-mode optical fibers by moving at least one of the polishing base and the multi-fiber optical connector while maintaining the contact between the tip ends of the multi-mode optical fibers and the upper surface of the film.
 2. The method of polishing an end face of a multi-fiber optical connector according to claim 1, wherein the soft material is composed of a spongy porous substance.
 3. The method of polishing an end face of a multi-fiber optical connector according to claim 2, wherein at least the upper surface of the film is made coarse by a surface processing.
 4. The method of polishing an end face of a multi-fiber optical connector according to claim 3, wherein the polishing material is applied to said coarsened upper surface of the film.
 5. The method of polishing an end face of a multi-fiber optical connector according to claim 4, wherein the soft material is a sponge pad.
 6. The method of polishing an end face of a multi-fiber optical connector according to claim 5, wherein the soft material has a thickness of approximately 5 mm.
 7. The method of polishing an end face of a multi-fiber optical connector according to claim 4, wherein the film is a polyethylene terephthalate film (PET film).
 8. The method of polishing an end face of a multi-fiber optical connector according to claim 7, wherein the PET film has a thickness of approximately 25 μm.
 9. The method of polishing an end face of a multi-fiber optical connector according to claim 4, wherein the polishing material has an average grain diameter of 0.5 μm or less.
 10. The method of polishing an end face of a multi-fiber optical connector according to claim 1, wherein the first step comprising: a step of mounting a soft material having a Shore hardness of less than 30 on a polishing base of a polishing machine; a step of mounting a film having a thickness of less than 75 μm without including any polishing material on the soft material; and a step of supplying a polishing material on an upper surface of the film.
 11. The method of polishing an end face of a multi-fiber optical connector according to claim 1, wherein in the third step, the polishing base moves so that the center axis line of the polishing base describes a circular locus while keeping a rotation-preventing attitude.
 12. The method of polishing an end face of a multi-fiber optical connector according to claim 1, wherein in the third step, the multi-fiber optical connector moves so that the center axis line of the multi-fiber optical connector describes a circular locus while keeping a rotation-preventing attitude.
 13. The method of polishing an end face of a multi-fiber optical connector according to claim 1, wherein the end face for connection of the multi-fiber optical connector is polished into a flat surface in advance and the tip ends of the multi-mode optical fibers project by a predetermined length from the end face for connection.
 14. A method of polishing an end face of a multi-fiber optical connector, the method including: a first step of mounting a sponge pad having a Shore hardness of less than 30 on a polishing base of a polishing machine; a second step of mounting a polyethylene terephthalate film (PET film) having a thickness of less than 75 μm without any polishing material on the sponge pad; a third step of supplying an polishing material on the upper surface of the PET film; a fourth step of arranging a multi-fiber optical connector having a plurality of multi-mode optical fibers so that the tip ends of the multi-mode optical fibers come into contact with an upper surface of the film with a predetermined pressure; and a fifth step of polishing the tip ends of the multi-mode optical fibers by moving at least one of the polishing base and the multi-fiber optical connector while maintaining the contact between the tip ends of the multi-mode optical fibers and the upper surface of the PET film.
 15. The method of polishing an end face of a multi-fiber optical connector according to claim 14, wherein the sponge pad has a thickness of approximately 5 mm.
 16. The method of polishing an end face of a multi-fiber optical connector according to claim 14, wherein the PET film has a thickness of approximately 25 μm.
 17. The method of polishing an end face of a multi-fiber optical connector according to claim 14, wherein the polishing material has an average grain diameter of approximately 0.5 μm or less. 